Insulin-like growth factor-1 activation of extracellular signal-related kinase-1 and -2 in growth hormone-secreting cells

Abstract

Intracellular pathways mediating feedback regulation by insulin-like growth factor-1 (IGF-1) of pituitary GH gene expression remain incompletely understood. Extracellular signal-related kinases (ERKs), a family of serine/threonine kinases, are activated by tyrosine kinase-associated growth factor receptors. To further define the IGF-1 postreceptor events occurring in GH-secreting cells, we investigated the activity of ERKs in response to IGF-1 in GC cells following stable transfection with either wild type human IGF-1 receptor cDNA (WT cells) or a mutant cDNA encoding a truncated, kinase-defective IGF-1 receptor with a dominant negative effect on endogenous receptor function (952STOP cells). Zymography of immunoprecipitated ERKs in myelin basic protein (MBP)-containing polyacrylamide gels demonstrated dose-dependent induction of ERK-1 and -2 activity by IGF-1 in GC cells with maximal activity occurring at 6 min. IGF-1-induced ERK activity in WT-transfected cells was up to 80-fold basal and 4-fold that observed in GC cells. 952STOP cells expressing the tyrosine kinase-deficient receptor were refractory to IGF-1 action, demonstrating minimal ERK induction. In contrast, 12-O-tetradecanoylphorbol 13-acetate stimulated ERK activity to the same degree in all three cell types regardless of their IGF-I receptor status. Forskolin (50 microM), isobutylmethylxanthine (0.5 mM), and forskolin/isobutylmethylxanthine in combination attenuated IGF-1-induced ERK activity in WT cells by 54, 55, and 75% respectively. The rapid, dose-dependent, and IGF-1 receptor-dependent activation of ERKs and the attenuation of this effect by cAMP suggest an interrelated role for both molecules in IGF-1 signal transduction in GH-secreting cells.